Cluster: clarify node->slave may be NULL.
Cluster: clarify node->slave may be NULL.

louisliangjun/redis,ramonsnir/redis,ramonsnir/redis,louisliangjun/redis,splitice/redis,louisliangjun/redis,maodeyi/redis_lua,maodeyi/redis_lua,louisliangjun/redis,splitice/redis,splitice/redis,splitice/redis,maodeyi/redis_lua,ramonsnir/redis